1. Field of the Invention
The present invention generally relates to a communications system in which a plurality of nodes are connected via transmission lines. More particularly, the present invention is concerned with a network switching control method and system used in such a communications system.
2. Description of the Prior Art
Recently, there has been an increased demand for communications networks. It is required that communications services be continuously provided or interrupted during a short period when a failure has occurred in communications facilities or lines. Hence, it is necessary to switch to one or a plurality of protection route channels (channel or channels in a route) if a failure has occurred in a node or a communications line.
FIG. 1A shows a conventional high-bit-rate optical communications system. The system shown in FIG. 1A comprises optical terminal station repeaters 100-0, 100-1, . . . , 100-n (where n is an integer), optical terminal station repeaters 110-0, 110-1, . . . , 110-n, optical intermediate optical terminal station repeaters 102-0, 102-1, . . . , 102-n, transmission paths 103 using optical fiber cables, and switching controllers 101 and 111. The repeaters 100-0-100-n and repeaters 110-1-110-n are coupled to each other via the repeaters 102-0-120-n and transmission paths 103.
Active lines are formed by the repeaters 100-1-100-n, 102-1-102-n, and 110-1-110-n, and protection lines are formed by the repeaters 100-0, 102-0 and 110-0. The switching controller 101 selectively couples communications channels on the input side of the system shown in FIG. 1A with the repeaters 100-1-100-n or 100-0. Similarly, the switching controller 111 selectively couples communications channels on the output side of the system shown in FIG. 1A with the repeaters 110-1-110-n or 110-0.
Normally, communications take place via the active lines. If a failure has occurred in a route via the repeater 102-n, the switching controllers 101 and 111 respectively select the repeaters 100-0, 102-0 and 110-0, as shown in FIG. 1B. Hence, a route via the repeaters 100-0, 102-0 and 110-0 is activated. It is necessary to switch to the repeaters 100-0, 102-0 and 110-0 for a short period in order to continuously perform communications services without any interruption.
In the conventional systems, active lines and protection lines are provided in a route, which is a communications path provided by a cable including 10 bound optical fibers covered by an insulating coat. If such a route (cable) happens to be mistakenly cut during a maintenance operation, not only the active lines but also the protection lines are disconnected. In this case, the protection lines are no longer capable of functioning as protection lines. In the conventional technique, another available route is manually searched for, and hence the communications services are greatly degraded.
With the above in mind, in the conventional technique, nodes 2A-2I are coupled to each other via optical fibers 121 so that a network is constructed as shown in FIG. 2A, and a network management device 120 is coupled to the nodes 2A-2I via control lines 122, so that the switching operations of the nodes 2A-2I are controlled by instruction signals transferred from the network management device 120 via the control lines 122. For example, if the route between the nodes 2D and 2G is totally cut in a state where a communications channel 4C passing through the nodes 2A, 2D, 2G and 2H has been established, the node 2A is informed of the occurrence such a failure. In response to the instruction signal from the network management device 120, the switching is carried out so that a communications channel 5C passing through the nodes 2A, 2B, 2E and 2H is established.
The above switching is carried out as shown in FIG. 2B. The node 2A sends the node 2B a switching control instruction to establish the communications channel 5C. In response to receipt of the switching control instruction, the node 2B executes a necessary switching operation. After receiving a switching completion signal from the node 2B, the node 2A sends the node 2E a switching control instruction similar to that sent to the node 2B. The node 2B executes a necessary switching operation. After receiving a switching completion signal from the node 2E, the node 2A sends the node 2H a switching control instruction similar to the switching control instructions sent to the nodes 2B and 2E. In this manner, the node 2A generates the switching control instruction for each of the nodes and transfers it to each of the nodes. Hence, it takes a long time to complete the switching for establishing the communications channel 5C.
The switching control instructions respectively sent to the nodes are transferred by using a fixed signal frame format as shown in FIG. 3B. The fixed signal frame format is generated by a data multiplexer unit 130 shown in FIG. 3A. The fixed signal frame format shown in FIG. 3B includes information to be multiplexed with a main signal, the above information containing the switching control instruction, alarm information, line setting information, test control information and other information. A capacity of 512 kbps is assigned to the switching control instruction, and a capacity of 128 kbps is allotted to each of the alarm information and the test control information, A capacity of 256 kbps is allotted to the line setting information. Two blocks each having a capacity of 256 kbps are assigned to other information, The control information is contained in a 1792 kbps signal frame, which is multiplexed with the main signal.
The signal frame format shown in FIG. 3B is a fixed format, and the fixed line capacities are allotted to the line setting information and the test control information which are not always used. The switching control instruction is transferred together with all of the other information. Hence it is impossible to efficiently perform the route switching in response to the occurrence of a failure.